There is clinical evidence that the capacity of 1,25 dihydroxy vitamin D [1,25(OH)2D3], the active metabolite of vitamin D, to stimulate intestinal Ca2+ absorption declines with age. Male Fischer 344 rats will be used to answer the question "Why is there an age related decrease in intestinal responsiveness to 1,25(OH)2D3 with regard to Ca2+ absorption?". The capacity of 1,25(OH)2D3 to increase expression of specific components of the Ca2+ transport mechanism will be studied. These components include calbindin, which is a protein involved in Ca2+ movement across the cell, and the intestinal Ca2+ pump, which pumps Ca2+ out of the intestinal absorptive cell. The intestinal 24 OHase, which degrades 1,25(OH)2D3 and which may regulate the action of 1,25(OH)2D3 in the intestine, will also be studied. Rats which are 2, 6, 12, and 27 months old will be used to study 1,25(OH)2D3 action during maturation, adulthood, and senescence. To answer the key question, three hypotheses will be tested: a) there is decreased stimulation of Ca2+ absorption because there is decreased induction of key intestinal proteins by 1,25(OH)2D3 with age. The induction of calbindin, the Ca2+ pump, and the 24 Ohase protein by 1,25(OH)2D3 will be measured using immunological methods. The capacity of intestinal MRNA from each age group to be translated into specific proteins will be measured in vitro. The capacity of intestinal ribosomes from each age group to synthesize specific proteins will be determined; b) there is decreased stimulation of Ca2+ absorption because there is altered function of the intestinal proteins induced by 1,25(OH)2D3 with age. The capacity of calbindin to bind Ca2+, the capacity of the Ca2+ pump to pump Ca2+, and the capacity of the 24 Ohase to hydroxylate 1,25(OH)2D3 will be determined for each age group. Vitamin D receptor binding in each age group before and after 1,25(OH)2D3 treatment will be measured; and c) there is decreased stimulation of Ca2+ absorption because there are age related changes in the intestinal tissue itself. Using cultured intestinal segments from each age group, the induction of key transport proteins by 1,25(OH)2D3 will be studied in vitro. The effect of 1,25(OH)2D3 on the phosphorylation of the Ca2+ pump will also be investigated. Finally, the effects of known stimulators of 1,25(OH)2D3 action, such as phorbol ester and ketoconazole, will be determined in segments from each age group. These studies may suggest ways of increasing Ca2+ absorption and, thus, improving Ca2+ balance and maintaining bone mass in the elderly.